Phase change material pack

ABSTRACT

A phase change material (PCM) pack ( 1 ) for an air conditioning system comprises phase change material sealed between a first thermally conductive layer ( 2 ) forming a first outer surface of the PCM pack and a second thermally conductive layer ( 2 ) forming a second outer surface of the PCM pack. At least the first or second outer surface of the PCM pack takes the form of a substantially planar surface having defined therein a plurality of depressions ( 4 ) deviating from the planar surface towards the interior of the PCM pack in a direction perpendicular to the planar surface. The depressions improve heat transfer between the pack and an airflow passing over the surface of the PCM pack.

FIELD OF THE INVENTION

This invention relates to a phase change material (PCM) pack for an airconditioning system, and to an air conditioning system comprising astack of such packs.

BACKGROUND

Published patent applications WO 2009/101398, WO 2010/092391 and WO2010/092393 describe phase change material packs and air conditioningarrangements using the packs. The phase change materials use the latentheat property of material to store thermal energy so that the phasechange material can be frozen using cool night time air and then used tocool daytime air for air conditioning purposes, for example withindomestic and commercial buildings.

It is desirable to maximise the heat transfer between phase changematerial packs and an air flow. The present invention, at least inpreferred embodiments, seeks to address this desirability.

BRIEF SUMMARY OF THE DISCLOSURE

In accordance with the present invention there is provided a phasechange material (PCM) pack for an air conditioning system (orventilation system). The pack comprises phase change material sealedbetween a first thermally conductive layer forming a first outer surfaceof the PCM pack and a second thermally conductive layer forming a secondouter surface of the PCM pack. At least the first or second outersurface of the PCM pack takes the form of a substantially planar surfacehaving defined therein a plurality of depressions deviating from theplanar surface towards the interior of the PCM pack in a directionperpendicular to the planar surface.

Thus, in accordance with the present invention, the depressions in theouter surface of the PCM pack enhance the heat transfer between an airflow passing over the outer surface and the PCM. It has been found thatvortices are generated within the depressions that enhance heat flow.

Typically, both the first and second outer surfaces of the PCM pack eachtake the form of a substantially planar surface having defined therein aplurality of depressions deviating from the planar surface towards theinterior of the PCM pack in a direction perpendicular to the planarsurface. However, depressions may be provided on only one outer surfaceof the pack. The other outer surface of the pack may therefore besubstantially flat or may have protrusions or depressions providedthereon, which may take any other suitable shape.

In general, the PCM pack is substantially cuboidal having a length andwidth many times larger than its depth. In this way, the PCM pack formsa thin panel.

In embodiments of the invention, the plurality of depressions isarranged in a regular array over the outer surface. This is notessential and the depressions may be arranged substantially randomly.However, for reasons of aesthetics, effective use of the surface area ofthe PCM pack and air flow, a regular array is presently preferred.

The array of depressions may comprise parallel rows of depressionsarranged orthogonally to the direction of fluid flow over the PCM pack,which will typically by the longitudinal (length) direction of pack.Thus, the rows of depressions may extend in the width direction of thepack. The pitch of depressions within the row may be selected tomaximise heat transfer. The spacing between adjacent rows may beselected to maximise heat transfer. Adjacent rows of depressions may bestaggered, for example by half the pitch of the depressions, in order tomaximise the use of the surface area of the PCM pack.

The (or each) outer surface is typically provided with a large number ofdepressions. Thus, each outer surface of the pack provided with saiddepressions may comprise at least ten depressions, preferably at least50 depressions, more preferably at least 100 depressions, morepreferably 1000 depressions or more.

In order to maximise heat transfer, the depressions may occupy asubstantial proportion of the outer surface of the PCM pack. Thus, thedepressions may occupy more than 15% of the planar surface area of the(or each) outer surface, possibly more than 25%, preferably more than50%, more preferably more than 75%.

In embodiments of the invention, the outer surface of the PCM pack isprovided with a large number of relatively small depressions. Thus, theplanar surface area of the outer surface occupied by each depression maybe less than 5% of the total planar surface area of the outer surface,possibly less than 1%, or less than 0.5% or even less than 0.1%. Theplanar surface area of the outer surface occupied by each depression canbe considered as the footprint of the depression.

In order to generate a desirable vortex, the depressions are presentlypreferred to be relatively shallow compared to their footprint. Thus,the maximum depth of each depression in the direction perpendicular tothe planar surface of the PCM pack (the depth direction) may be lessthan 75% of the square root of the planar surface area of the outersurface occupied by the depression, possibly less than 50% or even lessthan 25%.

However, in order to be effective in generating vortices, thedepressions should not be too shallow. Thus, the maximum depth of eachdepression in the direction perpendicular to the planar surface of thePCM pack may be greater than 10% of the square root of the planarsurface area of the outer surface occupied by the depression, possiblygreater than 20% or even greater than 30%.

In order that that the depressions create effective vortices, it isdesirable for the cross section of each depression in a planeperpendicular to the planar surface of the pack to form a smooth curve.The curve may be circular, elliptical, hyperbolic or any other suitableshape.

The footprint of the depressions may be any suitable shape, for exampleelliptical or polygonal. In embodiments of the invention, thedepressions have a substantially circular shape in the plane of theplanar outer surface. This provides a configuration that is easy tomanufacture.

Thus, in a presently preferred embodiment, the depressions are definedby a substantially spherical surface. For example, the depressions maybe substantially hemispherical or may be provided by less than half asphere.

The phase change material may comprise at least one of a salt hydrate,urea or paraffin. Other phase change materials may be used, includingmixtures. In particular, organic phase change materials made fromvegetable products may be used, such as the materials sold under thePure Temp trade mark by Entropy Solutions, Inc. of Plymouth, Minn., USA.Particular phase change materials are mixtures of paraffin and salthydrates such as the SP Blend manufactured by Rubitherm Gmbh of Berlin,Germany.

The salt hydrate may comprise a hydrate of sodium sulphate and/or ahydrate of calcium chloride. For example, the salt hydrate may be sodiumsulphate decahydrate, calcium chloride hexahydrate, calcium chloridetetrahydrate, calcium chloride dehydrate or a mixture of two or more ofthese. Other suitable salt hydrates are hydrates of sodium thiosulphate,sodium acetate, disodium hydrogen phosphate or sodium carbonate orsuitable mixtures of these and other salt hydrates.

The phase change material may comprise a hydrate of sodium sulphate andbetween 0 and 15% by weight of sodium chloride. The sodium chloride canbe used to lower the melting point of the sodium sulphate hydrate to therequired level. Similarly, the phase change material may comprise ahydrate of calcium chloride and between 0 and 15% by weight of potassiumchloride, sodium chloride and/or ammonium chloride.

Typically, the phase change material has a melting point between −15 and100 degrees centigrade, preferably between 15 and 40 degrees centigrade,more preferably between 20 and 30 degrees centigrade. The phase changematerial may be employed in a pack for refrigeration with a meltingpoint between −5 and 15 degrees centigrade or for a heating system witha melting point in excess of 40 degrees centigrade. The minimum andmaximum temperatures in the preceding ranges may be used interchangeablyto define the melting point range for the phase change material.

The thermally conductive material may be a metal, such as aluminium orstainless steel, which may be provided with an anti-corrosion coating.Alternatively, the thermally conductive material may comprise ametal-plastics composite, for example aluminium-coated plastics or HDPEwith an additive to increase thermal conductivity. The depressions maybe formed in the thermally conductive material by moulding, pressing,machining or any other suitable process.

The invention extends to an air conditioning system comprising aplurality of the PCM packs arranged in a stack with spacing betweenadjacent packs to allow air flow over the outer surface of each pack.The spacing of the packs is selected to minimise the pressure dropacross the stack while maximising heat transfer.

In order to generate the desired air flow through the stack, the maximumdepth of each depression in the direction perpendicular to the planarsurface of each PCM pack may be less than 90% of the spacing betweenadjacent packs, feasibly less than 75%, possibly less than 50%, possiblyless than 40%, or even less than 25%. In general, the maximum depth ofeach depression in the direction perpendicular to the planar surface ofeach PCM pack is greater than 25% of the spacing between adjacent packs.

An advantage of the depressions in the PCM pack is in reducing thethickness of the pack periodically to improve conductivity and benefitthe melting and freezing process.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are further described hereinafter withreference to the accompanying drawings, in which:

FIG. 1 shows a PCM pack according to an embodiment of the invention;

FIG. 2 shows schematically an air conditioning system according to anembodiment of the invention;

FIG. 3 shows in cutaway the detail of a PCM pack according to anembodiment of the invention;

FIG. 4 shows the airflow over the surface of the PCM pack of FIG. 3;

FIG. 5 shows schematically the configuration of the PCM pack of FIG. 3;

FIG. 6 shows four exemplary arrangements of dimpled panels;

FIG. 7 shows the heat transfer coefficient for the panels of FIG. 6 at arange of flow rates;

FIG. 8 shows the heat transfer coefficient for the panels of FIG. 6 at arange of pressure drops; and

FIG. 9 is a comparison of the heat transfer coefficient for selectedones of the panels of FIG. 6 at a range of pressure drops.

DETAILED DESCRIPTION

FIG. 1 shows a phase change material (PCM) pack 1 according to anembodiment of the invention. The PCM pack is formed from two layers 2 ofthermally conductive material sealed together at their edges to enclosethe PCM. The layers of thermally conductive material are also connectedat, in this embodiment, two connecting locations 3 within the footprintof the PCM pack 1. The connecting locations 3 take the form of a rivetwithin a depression formed in one layer 2 of thermally conductivematerial which rivet connects the two layers and provides mechanicalstability to the PCM pack 1.

As shown in FIG. 1, the surface of the PCM pack 1 is provided with aregular array of depressions 4 forming a surface texture of the PCMpack. The significance of the depressions 4 will be described in moredetail with reference to the subsequent Figures. In FIG. 1, thedepressions 4 are shown with a substantially rectangular outline.However, the depressions 4 may be circular in outline, as shown forexample in FIG. 3. Other outlines are also possible, for exampleelliptical or polygonal.

FIG. 2 shows the PCM packs 1 of FIG. 1 in a simple air conditioningsystem. The PCM packs 1 are arranged in a stack with spacing for airflowbetween adjacent packs 1. A fan 5 forces air through the stack of PCMpacks 1 from an air inlet 6 to an air outlet 7. An inlet valve 8 selectsthe source of air for the air inlet 6, which may be air from within abuilding or air from outside a building. Similarly, an outlet valve 9routes the air from the air outlet 7.

The air conditioning system is configured to use cool night time airfrom the outside of a building to freeze the PCM in the PCM packs 1 sothat during the day air can be cooled by being forced over the PCM packs1 by the fan 5 causing heat to be transferred from the air to melt thePCM. The air inlet valve 8 allows the air to be cooled to be selectedfrom outside air or, preferably, cooled air from within the building.

FIG. 3 shows the detail of a PCM pack 1 according to an embodiment ofthe invention in which the depressions 4 have a circular outline and arearranged in a regular array. In this embodiment, the depressions 4 aresubstantially hemispherical. However, other shapes of the depressions 4are possible, for example the depressions may be ovoid in shape. As canbe seen in FIG. 3, the PCM pack is provided with depressions 4 on bothmajor surfaces of the pack 1.

FIG. 4 shows the flow profile of air passing over the surface of the PCMpack 1 in the direction of the uppermost arrows in the Figure. As can beseen in FIG. 4, the provision of a depression 4 in the surface of thePCM pack 1 causes a vortex to be generated in the depression 4, whichhas been found to increase thermal transfer between the PCM pack 1 andthe passing air. This improves the efficiency of the PCM pack 1 whencompared to a PCM pack having only a flat surface.

FIG. 5 shows the dimensions of the arrangement of depressions 4 on thesurface of the PCM pack 1. As shown in FIG. 5, the substantiallyhemispherical depressions 4 are arranged in staggered rows orthogonal tothe direction of airflow (left to right in FIG. 5). The spacing betweenaligned rows is indicated by the parameter S. The diameter of eachdepression is indicated by the parameter d and the spacing of thecentres of depressions in each row (pitch) is indicated by the parameterp. As shown in FIG. 5, adjacent rows of depressions 4 have a relativedisplacement of p/2 in the direction normal to the flow direction. Thespacing between adjacent PCM packs in a stack is indicated by theparameter H, i.e. the height of the airflow passages between the PCTpacks 1. The depth of the depression relative to the surface of the PCMpack 1 is indicated by the parameters Hd.

Presently preferred embodiments of the invention have the followingranges of ratios of parameters:

-   -   0.2<H/d<1.5    -   0.1<Hd/d<0.3    -   0.25<d/S<0.57    -   0.35<S/p<2.0

EXAMPLES

Four dimpled layouts were selected for testing as shown in FIG. 6. Thespacing between adjacent PCM packs in this example was 8 mm. Thecharacteristics of the selected layouts are shown in the below.

Layout d Hd S Reference 6.9 mm    2 mm 13.9 mm Option 1 10 mm 2.1 mm34.6 mm Option 2 12 mm 2.6 mm 34.7 mm Option 3 15 mm 3.1 mm 34.6 mm

For testing purpose the selected dimpled layouts were machined on asingle side of 8 mm thick aluminium plates of 450×150 mm (FIG. 4). A setof two panels for each dimple layout was manufactured to create thedimpled channels. The side edges were sealed with an insert of a purposemade T-profile to set the channel height at the required level (8 mm or10 mm). The back of the dimpled plate was attached to a thermoelectriccooling assembly that allowed control of the wall temperature of thecooling channel to the required level. A purpose built test rig was usedto evaluate the dimpled channel performance. The test channel wasconnected to a pressurised chamber via a pre-conditioning passage withsmooth walls. The air temperature in the chamber was regulated withhalogen bulbs connected to a PID controller to achieve the requiredinlet temperature at the test section. The air flow rate was suppliedusing variable speed DC fans to the pressurised chamber and the rate offlow was measured upstream of the supply fan with a standard inlet conedevice. Air temperature and relative humidity were evaluated at bothinlet and outlet of the test section. The plate temperature was measuredat six different locations with thermocouple probes inserted into 40 mmlong channels drilled in from the lateral side of the plates. Thechannels were filled with thermo-conductive paste to ensure good contactbetween the temperature probes and the body of the dimpled plate.

Each dimple layout was tested at various flow rates to evaluate thethermal performance under different flow regimes. Two sets ofmeasurements were conducted. The first set of measurements was focusedon comparing the dimpled layouts against a smooth wall channel, using 8mm channel height. The second round was comparing the best performingdimpled plate with an existing PCM panel. The dimpled channel height wasincreased to 10 mm for the subsequent measurements.

As shown in FIG. 7, the heat transfer coefficient (h_(tr)) of thedimpled plates showed a considerable increment of about 20% compared tothe smooth plate in the laminar rage. As the flow regime altered towardsa more turbulent range the h_(tr) in the dimpled channels also increasedat a steady rate up to 80 to 90% higher than within the smooth channel.The “Reference” and Option 3 dimples contributed to the highestincrement in the heat transfer. Option 2 and Option 3 layouts have alsofollowed the trend of the h_(tr) improvement, but at a noticeably lowerrate.

FIG. 8 shows that the heat transfer performance of each panel follows awell defined relationship with the specific pressure loss regardless ofdesign. However, the amount of air (or face velocity) required toachieve a certain heat transfer level at a given pressure maysubstantially differ between various designs. As an example 25 W/m²Kh_(tr) is achievable with each design at around 20 Pa pressure loss, butthe amount of air needed in case of the flat plate may be up to 5.5 l/swhile in the case of the Option 3 layout only about 3.5 l/s.Consequently, the dimpled layout can be matched to achieve the desiredheat transfer and pressure loss for the amount of air that is suppliedthrough the thermal battery (PCM pack).

The ambient temperatures during the second set of measurements wereconsiderably cooler at 17-20° C. Inlet temperatures were set within the27-30° C. range and the channel wall temperatures established between 20and 23° C. For this set of measurement the ‘Option 3’ layout wasselected as it was performing in the top range compared to the otherinvestigated layouts. The measurements were carried out at both 8 mm and10 mm channel height. Furthermore, the flat wall channel was tested as areference case and a single channel made of 2 PCM panels but of equalwidth as the dimpled channel (150 mm) was also investigated. FIG. 9shows the heat transfer coefficient for each plate against the pressuredrop across the channel. As can be seen from FIG. 9, the Option 3configuration had superior heat transfer for a given pressure drop.

In summary, a phase change material (PCM) pack 1 for an air conditioningsystem comprises phase change material sealed between a first thermallyconductive layer 2 forming a first outer surface of the PCM pack and asecond thermally conductive layer 2 forming a second outer surface ofthe PCM pack. At least the first or second outer surface of the PCM packtakes the form of a substantially planar surface having defined thereina plurality of depressions 4 deviating from the planar surface towardsthe interior of the PCM pack in a direction perpendicular to the planarsurface. The depressions improve heat transfer between the pack and anairflow passing over the surface of the PCM pack.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of them mean “including but notlimited to”, and they are not intended to (and do not) exclude othermoieties, additives, components, integers or steps. Throughout thedescription and claims of this specification, the singular encompassesthe plural unless the context otherwise requires. In particular, wherethe indefinite article is used, the specification is to be understood ascontemplating plurality as well as singularity, unless the contextrequires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith. All of the features disclosed in this specification(including any accompanying claims, abstract and drawings), and/or allof the steps of any method or process so disclosed, may be combined inany combination, except combinations where at least some of suchfeatures and/or steps are mutually exclusive. The invention is notrestricted to the details of any foregoing embodiments. For example, itis possible for the packs of the invention to be used in a fluidconditioning system utilising fluids other than air. The inventionextends to any novel one, or any novel combination, of the featuresdisclosed in this specification (including any accompanying claims,abstract and drawings), or to any novel one, or any novel combination,of the steps of any method or process so disclosed.

1. A phase change material (PCM) pack for an air conditioning system,the pack comprising phase change material sealed between a firstthermally conductive layer forming a first outer surface of the PCM packand a second thermally conductive layer forming a second outer surfaceof the PCM pack, wherein at least the first or second outer surface ofthe PCM pack takes the form of a substantially planar surface havingdefined therein a plurality of depressions deviating from the planarsurface towards the interior of the PCM pack in a directionperpendicular to the planar surface.
 2. A PCM pack as claimed in claim1, wherein both the first and second outer surfaces of the PCM pack eachtake the form of a substantially planar surface having defined therein aplurality of depressions deviating from the planar surface towards theinterior of the PCM pack in a direction perpendicular to the planarsurface.
 3. A PCM pack as claimed in claim 1, wherein the plurality ofdepressions is arranged in a regular array over the outer surface.
 4. APCM pack as claimed in claim 1, wherein each outer surface of the packprovided with said depressions comprises at least ten depressions.
 5. APCM pack as claimed in claim 1, wherein the depressions occupy more than15% of the planar surface area of the outer surface provided with saiddepressions.
 6. A PCM pack as claimed in claim 1, wherein the planarsurface area of the outer surface occupied by each depression is lessthan 5% of the total planar surface area of the outer surface.
 7. A PCMpack as claimed in claim 1, wherein the maximum depth of each depressionin the direction perpendicular to the planar surface of the PCM pack isless than 75% of the square root of the planar surface area of the outersurface occupied by the depression.
 8. A PCM pack as claimed in claim 1,wherein the maximum depth of each depression in the directionperpendicular to the planar surface of the PCM pack is greater than 10%of the square root of the planar surface area of the outer surfaceoccupied by the depression.
 9. A PCM pack as claimed in claim 1, whereinthe cross section of each depression in a plane perpendicular to theplanar surface of the pack forms a smooth curve.
 10. A PCM pack asclaimed in claim 1, wherein the depressions have a substantiallycircular shape in the plane of the planar outer surface.
 11. A PCM packas claimed in claim 1, wherein the depressions are defined by asubstantially spherical surface.
 12. A PCM pack as claimed in claim 1,wherein the phase change material comprises at least one of a salthydrate, urea or paraffin.
 13. A PCM pack as claimed in claim 1, whereinthe phase change material has a melting point in the range −5 to 35degrees centigrade.
 14. A PCM pack as claimed in claim 1, wherein thethermally conductive material comprises aluminium or stainless steel.15. A PCM pack as claimed in claim 1, wherein the thermally conductivematerial comprises a metal-plastics composite.
 16. An air conditioningsystem comprising a plurality of PCM packs as claimed in claim 1arranged in a stack with spacing between adjacent packs to allow airflow over the outer surface of each pack.
 17. An air conditioning systemas claimed in claim 16, wherein the maximum depth of each depression inthe direction perpendicular to the planar surface of each PCM pack isless than 75% of the spacing between adjacent packs.
 18. An airconditioning system as claimed in claim 16, wherein the maximum depth ofeach depression in the direction perpendicular to the planar surface ofeach PCM pack is greater than 25% of the spacing between adjacent packs.